The long-term goal of this project has been to understand the structural organization of lipids in blood cell membranes, to examine how perturbations in membrane lipids cause cell dysfunction and human disease, and to attempt to modify disease states by exerting an influence on membrane structure. In this renewal application we propose to study the regulation of membrane lipids during the process of hematopoietic cell differentiation. We will utilize long-term tissue culture cell lines which are derived from human and murine acute leukemia and which possess the ability to differentiate in response to chemical stimuli. We will also obtain cells from patients with acute leukemia and study them under conditions in which they can be induced to enter into a differentiation program. Induction of differentiation will be viewed in terms of four separate stages: signal reception, signal transduction, commitment to differentiation, and expression of the differentiated state. Specific experiments will examine the binding of inducers to cell membranes, early changes in membrane lipid metabolism associated with signal transduction, changes in lipid synthesis and membrane biogenesis during the commitment of cells to differentiation, and modification of membrane lipid composition during completion of the differentiation program. We will analyze membrane fluidity throughout the differentiation process, using hydrophobic fluorescent probes, and we will relate the structural changes observed in this way to the biochemical events simultaneously occurring in the membrane. The immediate goal of these studies is to define the structural and chemical changes in membrane lipids associated with hematopoietic cell differentiation. The ultimate goal is to understand how to cause leukemic cells in man to enter into a differentiation program and thereby cease relentless proliferation.